1. Field of the Invention
The invention relates to a fuel cell and, more particularly, to a fuel cell in which a mixed fuel gas comprising a fuel gas, such as methane, and oxygen is fed to a fuel cell element or elements contained in a container to generate electricity, and the gas having passed through the fuel cell element or elements is discharged, as an exhaust gas, from the container.
2. Description of the Related Art
A fuel cell can be expected to have high efficiency, in power generation, compared to power generation in a thermal power plant or others, and is currently being studied by many researchers.
As shown in FIG. 4, such a conventional fuel cell is provided with an element 106 for the fuel cell, which element uses, as a solid electrolyte layer 100 of an oxygen ion conduction type, a fired body made of stabilized zirconia to which yttria (Y2O3) is added, the solid electrolyte layer 100 having one side on which a cathode layer 102 is formed, and another side on which an anode layer 104 is formed. Oxygen or an oxygen-containing gas is fed to the side of the cathode layer 102 of the fuel cell element 106, and a fuel gas, such as methane, is fed to the side of the anode layer 104.
The oxygen (O2) fed to the side of the cathode layer 102 of the fuel cell element 106 as shown in FIG. 4 is ionized into oxygen ions (O2−) at the boundary between the cathode layer 102 and the solid electrolyte layer 100, and the oxygen ions (O2−) are conducted to the anode layer 104 by the electrolyte layer 100. The oxygen ions (O2−) conducted to the anode layer 104 react with the methane (CH4) gas fed to the side of the anode layer 104, to thereby form water (H2O), carbon dioxide (CO2), hydrogen (H2), and carbon monoxide (CO). During the reaction, the oxygen ions release electrons, resulting in a potential difference between the cathode layer 102 and the anode layer 104. Accordingly, by establishing an electrical connection between the cathode layer 102 and the anode layer 104 by a lead wire 108, the electrons of the anode layer 104 pass in the direction toward the cathode layer 102 (the direction of the arrow in the drawing) through the lead wire 108, and electricity is produced by the fuel cell.
The fuel cell shown in FIG. 4 is operated at a temperature of about 1000° C. At such a high temperature, the side of cathode layer 102 of the fuel cell element 106 is exposed to an oxidizing atmosphere, and the side of anode layer 104 is exposed to a reducing atmosphere. Consequently, it has been difficult to enhance the durability of the fuel cell element 106.
It is reported, in Science, vol. 288, pp2031–2033 (2000), that, as shown in FIG. 5, even when a fuel cell element 206 made up of a solid electrolyte layer 200, and a cathode layer 202 and an anode layer 204 respectively formed on one side and another side of the electrolyte layer 200, is placed in a mixed fuel gas of methane and oxygen, the fuel cell element 206 develops an electromotive force.
By placing the element 206 in a mixed fuel gas, as in the fuel cell illustrated in FIG. 5, the element 206 can be enveloped as a whole in substantially the same atmosphere, and can have improved durability compared to the element 106 shown in FIG. 4 in which the respective sides of the element 106 are exposed to atmospheres different from each other.
Nevertheless, as a mixed fuel gas of methane and oxygen is fed to the fuel cell shown in FIG. 5, at a high temperature of about 1000° C., the mixed fuel gas is adjusted to contain oxygen at a concentration smaller than the ignition limit (lower ignition limit) concentration of oxygen for the mixture of methane and oxygen (a concentration of methane exceeding the ignition limit (upper ignition limit) concentration of methane for the mixture of methane and oxygen) prior to being fed to the fuel cell, in order to avoid the danger of an explosion.
For this reason, in the condition of an extremely small amount of oxygen, the fuel cell element 206 has a small power generation efficiency, and the fuel component, such as methane, may be carbonized to thereby further reduce the performance of the fuel cell.